Solenoids energized by Alternating Current (AC) are notorious for producing objectionable noise. The noise is emitted because the magnetic force holding the solenoid plunger to the core varies from some maximum to zero twice every current cycle. If the current is 60 Hz, then the magnetic force reaches a zero value 120 times per second. This magnetic field and force variation creates conditions which can lead to noise. As the solenoid usually has a return spring, one of the major noise contributors is the tendency of the plunger to leave the surface of the core if the magnetic force drops below the spring force. Then, when the magnetic force rises above the spring force the plunger returns to the core with an impact and some of this energy is converted to audible noise energy. Other noise contributions come from the general instability of the plunger within the vibrating magnetic field. The plunger can move uncontrollably and impact nearby surfaces and make noise as a result.
The solenoid device is well known, and many schemes have been proposed and applied to reduce or remove the noise problem. For example, an old and presently used technique is to use a electrically conductive “shading ring”. This is a small secondary single winding in which current flow is induced from the main magnetic field. This current flow produces a small magnetic force that is sufficient to maintain secure contact between the core and plunger against the return spring force during the time that the main magnetic force falls below the spring force.
This scheme works well, but as the force generated by the small current flow in the shading ring is only enough to hold the spring if the plunger and core are kept in intimate contact, and stable, anything that changes the magnetic resistance, such as a small gap between the plunger and core, will render the shading ring ineffective. Consequently, the solenoid will create an undesired buzzing sound.
Also, if a small burr, dent on a contact surface, or debris is present on the mating surfaces of either the plunger or the core, a pivot axis will be created about two points of contact. This allows freedom of movement within the constraint of the guiding tube. Not only does this introduce a gap and lower the permeability of the magnetic circuit, but it allows the magnetic field to move the body of the plunger transversely. This type of magnetic-field-induced buzzing is observed in the absence of a return spring.
In an operating solenoid, if the gap between the core and plunger mating surfaces is sufficient to render the shading ring ineffective against the return spring, and this circumstance is combined with the presence of transverse magnetic field vibrations, the resulting noise and the force of part collisions can be worsened and lead to premature failure of the solenoid.
A low cost of production is needed for solenoids that are constructed for high volume manufacturing and sales. Low-cost construction leads to conditions that promote misalignment, the presence of debris, and low precision. The foregoing conditions can contribute to and/or cause unwanted gaps and noisy operation. So it is not surprising that efforts have been undertaken in the industry to make solenoid construction more precise. However, providing such precision is costly. And although such precision succeeds in producing a higher percentage of quiet solenoids, it does not solve the noise problem completely.
Solenoids serve many markets and end uses. Some require quick response to a signal input and a rapid return from an energized position. In such cases, the coercive force from the retained magnetism must be very low. Such low retained magnetism can be achieved by using magnetic materials in the core and plunger that are soft. Ensuring that the parts stay soft enough, for the desired purpose, during the cold-working of manufacturing is difficult and may require inter-process annealing. Such inter-process annealing increases the cost of production.
Some solenoids are not used where speed of action is important. In these cases, the parts may be harder and increased coercive force may be allowed. However there are limits. Even if it does not matter if a valve shuts several seconds after the power is cut off, it gives the impression of poor control and impending failure. So usually, the return spring force is made sufficient to overcome the coercive force. But this increases the likelihood of generating an undesired buzzing sound, discussed above.